An official period of state mourning is underway in the UK on Friday, a day after Queen Elizabeth II died, and there has been an outpouring of grief from members of the public who have been laying flowers at royal residences around the country for its longest-reigning monarch.

In London, 96 rounds of gun salutes — one for every year of the Queen’s life — have been fired in tribute to the late monarch from the Tower of London and Hyde Park by the King's Troop Royal Horse Artillery and the Honourable Artillery Company.

Churches also tolled their bells, after the Church of England sent out guidance to parishes, chapels and cathedrals across the country encouraging them to open for prayer or special services. Major sporting events, including football and cricket matches, have been postponed as a mark of respect.

British Prime Minister Liz Truss led tributes at a special joint Parliament session convened to pay tribute to the Queen, who died at her Balmoral Castle residence in Scotland on Thursday.

The special session is expected to last through the day until 10pm local time. The regular business of government is at a halt, unless anything urgent occurs, with the focus to fall entirely on the Queen during the 10-hour sitting of Parliament.

Truss said that since the death of the Queen was announced there has been the "most heartfelt outpouring of grief" across the world.

"Everyone who met her will remember the moment. They will speak of it for the rest of their lives. Even those who never met her, her late majesty's image is an icon for what Britain stands for as a nation," she said, reiterated her earlier statement describing the late monarch as the “rock on which modern Britain was built”.

With reference to the Queen's heir King Charles III, she told the Commons that the nation will offer its "loyal service to our new King".

Truss and senior ministers are scheduled for public service of remembrance at St Paul's Cathedral in central London and then the government is due to confirm the length of national mourning, which is likely to be around 12 days, from now up to the day after the Queen's funeral.

The funeral day will be a public holiday in the form of a Day of National Mourning.